1. Field of the Invention
The invention relates to a nickel plating solution and its preparation method, a nickel plating method and printed wiring board copper foil. In particular, the invention relates to a nickel plating solution and its preparation method, a nickel plating method and printed wiring board copper foil, taking account of reducing the load on the environment.
2. Description of the Related Art
Generally, used as a printed wiring board conductor is copper or copper alloy foil (herein referred to as simply “copper foil”) to which various surface treatments are made. Particularly, in the field of flexible printed wiring boards, to fabricate a printed wiring board, the copper foil is laminated with a polyimide-based resin film, or is coated with a varnish comprising mainly polyamic acid that is a precursor of polyimide-based resin. Herein, the polyimide-based resin film or varnish or cured varnish used in this case, may be referred to as “printed wiring board (PWB) base material” or simply “base material”.
The good adherence (high peeling strength) between the copper foil and PWB base material is required. To this end, the bonded side of untreated copper foil (herein, simply “original foil” or “untreated foil”) that is a raw material, is often made uneven by etching or plating. This surface treatment is called “roughening”.
The original foil (or untreated foil) is classified into electrolytic copper foil and rolled copper foil according to its fabrication methods, but both of them use the same roughening method. For example, as the general method, there is the method depositing (precipitating) rice-grain-shaped copper on copper foil surface by burnt plating, or the method selectively etching crystal grain boundaries using the acid.
With respect to the roughening by burnt plating, besides general copper plating (copper sulfate plating), roughening by copper-nickel alloy plating has also been developed (see for example, JP-A-52-145769).
The lamination of the flexible printed wiring board copper foil and polyimide-based resin film is performed typically by heating to the order of 300° C. This temperature condition is a high temperature range for the copper foil. Such a high temperature range tends to cause active movement (diffusion) of copper atoms in the copper foil towards the laminated polyimide-based resin film. The diffused copper atoms cause decomposition of the polyimide-based resin, and produce a fragile layer, which results in a decrease in the adhering strength between the copper foil and the polyimide-based resin film. Also, the copper atom diffusion after wiring formation may cause dielectric breakdown and a short.
To prevent these malfunctions, particularly where a high temperature (e.g., order of 150-300° C.) step is included in the manufacturing process, nickel or nickel alloy plating is applied to the roughened copper foil. This is considered to be because the nickel atoms, whose diffusion coefficient is small (whose activation energy for diffusion is high) compared with the copper atoms and whose movement in the same temperature range is therefore small, can be present as a copper atom diffusion barrier. Also, applying that plating allows enhancement of resistance to oxidative and damp discoloration from the point of view of appearance.
Generally, the nickel plating application conventionally uses a nickel plate or a nickel chip as the anode. Such nickel plating causes the anode (nickel plate) to be dissolved in the plating solution to feed nickel ions, and therefore inevitably causes shifts with time, for example in the anode shape due to the consumption, the distance between the anode and plated member, etc., which results in difficulty in long-term stable nickel plating in the same setting conditions. Also, frequent replacement of the consumed anode is needed, which therefore causes a decrease in productivity. Some plating apparatus structure may require complicated anode replacement.
To solve the above drawbacks, in recent years, plating using an insoluble anode, which is often performed in copper sulfate plating, etc., has been developed for nickel plating.
In the nickel plating using an insoluble anode, however, an organic substance added as a brightener or a surfactant tends to be decomposed by oxygen generated at the anode, which causes difficulty in long-term maintenance of solution composition.
For this reason, JP-A-11-200099 discloses that an anode is covered with an ion-exchange membrane, thereby avoiding contact between the anode and the additive (organic substance) to suppress organic substance decomposition.
Also, various plating solution compositions in the nickel plating solution has so far been developed, but most generally used is a composition called “Watt bath” comprising nickel sulfate, nickel chloride and boric acid.
The basic principle of the nickel plating solution is that in the plating solution, current is conducted between an anode and a cathode to be plated, thereby depositing (precipitating) nickel ions in the plating solution on the cathode to be plated. In this case, chloride ions in the plating solution facilitate nickel dissolution of the anode, and the nickel ions are thereby fed. In that case, to restrain variation in pH of the plating solution, boric acid is generally added as a pH buffer. As an alternative of the boric acid as the pH buffer, a nickel plating method using citric acid is also known (see for example, JP-A-2001-172790).
As a nickel alloy plating, on the other hand, nickel-cobalt alloy plating is known (see for example, JP-A-6-54829).
It is known that cobalt is added in a nickel plating solution to produce nickel-cobalt alloy plating, to enhance barrier and discoloration-resistant properties of the nickel plating solution, and therefore alkali etching property, in comparison with simple nickel plating.
Additionally, cobalt is considered to also serve as a catalyst to activate a reaction of polyimide-based resin when laminated with a polyimide-based resin film, and contribute to enhancement in the peeling strength.
Besides the nickel plating, to the PWB copper foil are applied zinc plating, chromate and silane coupling (see for example, JP-A-2005-8972), although not describing in detail herein.
In the conventional nickel plating using an insoluble anode, however, it is difficult, as mentioned above, to maintain the solution composition for a long period of time, and to apply continuously stable plating because its pH tends to be lowered by oxygen produced at the anode. Particularly, in plating the flexible PWB copper foil, the spacing between the anode and the cathode is, in many cases, made as small as possible to decrease the voltage and thereby have the economical effect (a decrease in manufacturing cost), which may actually cause a substantial local drop in pH in its plated portion. In this case, in the PWB copper foil having a very small absolute value of deposited metal (very thin plating thickness) to be controlled, even a local (overall slight) pH variation leads to a very large variation relatively (relative to intended plating thickness).
Also, above-mentioned JP-A-11-200099 is complicated in its apparatus structure in comparison with a plating method using a soluble anode, and therefore has the disadvantage from the point of view of initial cost (facility introduction cost) and running cost (operation cost including maintenance cost).
Also, the boric acid conventionally used as the pH buffer is said to have difficulty obtaining a stable nickel plating film due to its deficient pH buffering ability closely to copper foil surface using an insoluble anode. Further, the boric acid is known as a toxic substance to human body, and has in recent years been restricted from being drained. The prime task is to develop an apparatus for securely removing boric acid in drainage and a pH buffer to be substituted for the boric acid.
Although above-mentioned JP-A-2001-172790 discloses the pH buffer to be substituted for the boric acid, the plating solution composition disclosed in JP-A-2001-172790 contains a nickel chloride and therefore, where an insoluble anode is used, tends to produce chlorine gas from the anode, which would make the apparatus and work environment be in a very poor state (severe condition).
Also, above-mentioned JP-A-6-54829 discloses the nickel-cobalt alloy plating, but neither supply sources of the nickel and cobalt nor a definite precipitation (electrodeposition) ratio thereof, and there is therefore no assurance of reducing the load on the environment and achieving the desirable adherence to a PWB base material.